[Ada] Fix internal error on subprogram instantiation with -gnatzc

[Ada] Fix internal error on subprogram instantiation with -gnatzc

This fixes a fallout of the recent change keeping the
Is_Generic_Instance flag on the wrapper package built for the
instantiation of a generic subprogram.

There is no need to visit the Instance_Spec of an
N_Subprogram_Instantiation node anymore because the regular processing
for an N_Package_Declaration node now does the job for instantiations of
generic subprograms.

The following subprogram must compile again quietly with -gnatzc:

with Gen_Proc;

package RCI is
   pragma Remote_Call_Interface;

   procedure Inst_Proc is new Gen_Proc;

   procedure P (S : String);
end RCI;

generic
procedure Gen_Proc (S : String);
pragma Remote_Call_Interface (Gen_Proc);

with Ada.Text_IO; use Ada.Text_IO;
procedure Gen_Proc (S : String) is
begin
   Put_Line ("Gen_Proc called: " & S);
end Gen_Proc;

2019-08-19  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_dist.adb (Build_Package_Stubs): Do not specifically visit
	the declarations of an N_Subprogram_Instantiation node.

From-SVN: r274647